In Ireland (as in most countries) you need to enquire if you need planning permission before any new building work can commence on your site. There is a limited amount of building work which would be exempt from requiring planning permission. Exempt works are generally extensions to the rear of your house measuring less than 40m2 (400ft2 with some allowances made for a 2 storey portion). The specifics will depend on your local council.

The first step is to engage a designer to draw up the plans of your new home or extension. A competent designer will be able to advise you how to tailor your design in to improve your chances of a successful planning application.
The time it takes to design a house will vary depending on the client and the complexity of the design but normally takes upwards of 6-8 weeks. Once submitted, the planning authority have 8 weeks to decide on your application. If they grant you permission then you have a further 4 weeks to wait for appeals of your grant. Assuming no appeals are lodged you can now begin building works on site (once you have complied with the conditions of the new building regulations)

When thinking about your new home, particularly on a green field site, it is important to bear in mind what your local authority favour in terms of design approach.
Cork County Council have published an excellent document called the Cork Rural Design Guide (corkcoco.ie) outlining their guidelines in this regard and it is a must read for those embarking on this exciting journey.

Beyond this it is clearly important to speak to your neighbours and people in your area to let them know what you hope to build. Being proactive at this point can avoid frustration and delay later. If someone has an issue with your proposal it’s better to know that at design stage rather than find it out once someone objects to your application. You will also get a sense of others’ experience of dealing with your local planning department and be better prepared before you lodge the plans of your dream home.

Best of luck!!!